Reland "Frame rate controller per spatial layer."

This is a reland of ae9e188e67a489db597224e3cfcfdee04edf0cba

Original change's description:
> Frame rate controller per spatial layer.
>
> This allows VP9 encoder wrapper to control frame rate of each spatial
> layer. The wrapper configures encoder to skip encoding spatial layer
> when actual frame rate exceeds the target frame rate of that layer.
> Target frame rate of high spatial layer is expected to be equal or
> higher then that of low spatial layer. For now frame rate controller
> is only enabled in screen sharing mode.
>
> Added unit test which configures encoder to produce 3 spatial layers
> with frame rates 10, 20 and 30fps and verifies that absolute delta of
> final and target rate doesn't exceed 10%.
>
> Bug: webrtc:9682
> Change-Id: I7a7833f63927dd475e7b42d43e4d29061613e64e
> Reviewed-on: https://webrtc-review.googlesource.com/96640
> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
> Reviewed-by: Erik Språng <sprang@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#24593}

TBR=sprang@webrtc.org

Bug: webrtc:9682
Change-Id: Idcce315890c79301da532f9ba4997e9606f73fb0
Reviewed-on: https://webrtc-review.googlesource.com/99340
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24669}

modules/video_coding/codecs/vp9/vp9_impl.h

@@ -117,8 +117,7 @@ class VP9EncoderImpl : public VP9Encoder {
   bool is_svc_;
   InterLayerPredMode inter_layer_pred_;
 
-  // Framerate controller.
-  FramerateController framerate_controller_;
+  std::vector<FramerateController> framerate_controller_;
 
   // Used for flexible mode.
   bool is_flexible_mode_;